Jet-torus interaction revealed by sub-parsec SO absorption in NGC 1052

TL;DR

This study uses very long baseline interferometry at λ2 mm with the KVN to spatially resolve SO absorption in the sub-parsec nucleus of NGC 1052. The 129 GHz continuum reveals a bright core plus a five-component two-sided jet, while the SO J_N = 3_3−2_2 absorption is detected mainly toward the central components and spans over $>700$ km s$^{-1}$, decomposed into three Gaussian features with a blueshifted component at $V_{ m ctr}-V_{ m sys} \,=\,-412$ km s$^{-1}$ and a redshifted one near $V_{ m sys}+223$ km s$^{-1}$. The SO gas is confined to a region smaller than $0.45$ pc from the SMBH and is interpreted as shock-evaporated material at the inner torus edge, entrained outward by the jet while some clumps fall back as inflow, indicating a clumpy, multiphase torus undergoing jet–torus interactions. Partial spatial/kinematic overlap with the 321 GHz H$_2$O maser suggests that jet–torus shocks may also excite H$_2$O masers, underscoring the role of shocks in shaping the circumnuclear molecular environment. This work demonstrates the power of sub-pc SO absorption as a tracer of jet–torus physics in AGNs and sets the stage for broader surveys of sulfur-bearing species in similar systems.

Abstract

We report the first λ2-mm very long baseline interferometry (VLBI) observations of the radio galaxy NGC 1052, conducted with the Korean VLBI Network (KVN) using a wide-band recording mode. Leveraging the wide bandwidth covering a velocity range at 2300 km/s, we successfully detect broad (> 700 km/s) multi-component SO J_N = 3_3 - 2_2 absorption against the sub-parsec-scale continuum structure. The absorption profile consists of both redshifted and blueshifted components, including a newly identified blueshifted feature at -412 km/s relative to the systemic velocity. Significant SO absorption is confined to the central components, with no substantial detection toward the outer jet components. This constrains the location of SO gas to a compact region smaller than 0.45 pc in the sub-parsec vicinity of the supermassive black hole (SMBH). Our results support the scenario in which SO molecules are evaporated through shock heating caused by jet-torus interaction. The SO gas clumps are likely driven outward by the jet, with some returning toward the SMBH as inflowing material. Comparison with 321 GHz H2O masers reveals partial similarities in spatial distribution and radial velocity, suggesting that the jet-torus interaction may also trigger the excitation of H2O masers.

Figures (5)

• Figure 1: Continuum map of the nuclear region in NGC 1052 at 129 GHz. Contours begin at 3 times $I_{\rm rms}$ and increase by a factor of 2, where $I_{\rm rms}$ = 1.42 mJy beam$^{-1}$. The peak intensity is 125 mJy beam$^{-1}$. The synthesized beam is 0.88 $\times$ 0.73 mas at a PA of 73$^{\circ}$ as represented by a cross-hatched ellipse in the bottom-left corner. The plus symbols mark the location of fitted Gaussian components E1, E2, Core, W2 and W1. The parameters obtained from the Gaussian components are listed in table \ref{['tab:cntgauss']}. Alt text: Contour map of the sub-pc region of NGC 1052 at 129 GHz, showing five labeled Gaussian components (E1, E2, Core, W2, and W1)
• Figure 2: (a)--(e) SO $J_N = 3_3-2_2$ absorption spectra integrated over a 0.3 $\times$ 0.3 mas$^2$ region at different locations of E1, E2, Core, W2 and W1, as marked by arrows. The zero flux level corresponds to the continuum level at each location. The velocity resolution is 37.1 km s$^{-1}$. (f) Continuum contour map of NGC 1052 at $\lambda$2 mm also shown in figure \ref{['fig:dcntmap']}. Alt text: Five panels showing SO line spectra at different locations (E1, E2, Core, W2, and W1), along with a continuum map of NGC 1052 indicating their spatial positions.
• Figure 3: Spectral profile of SO absorption integrated over three 0.3 $\times$ 0.3 mas$^2$ regions centered on the continuum components E2, Core, and W2. The spectrum is normalized by the combined continuum flux densities of E2, Core and W2. The velocity resolution is 55.6 km s$^{-1}$, and the rms noise level is 0.017 in normalized flux density. The three colored curves indicate a triple-Gaussian fit to the observed profile. The fitting results are listed in table \ref{['tab:sogauss']}. Alt text: A line graph displaying the SO absorption spectrum with three fitted components labeled Blue1, Blue2, and Red1.
• Figure 4: Color optical depth channel maps of three SO absorption components. (Top) Blue1 at 1095 km s$^{-1}$, (Middle) Blue2 at 1392 km s$^{-1}$, (Bottom) Red1 at 1730 km s$^{-1}$. They are overlaid by the continuum contour map of NGC 1052 at $\lambda$2 mm also shown in figure \ref{['fig:dcntmap']}. The centroid velocity of each component is shown at the upper right. The rms noise in optical depth is typically 0.2. Alt text: Three color panels comparing the optical depth of different SO absorption components.
• Figure 5: A schematic view of the sub-pc nuclear region in NGC 1052. The two-sided jet axis is inclined by $\ge 76^{\circ}$ with respect to the line of sight sss08. The torus has several phase layers of plasma and molecules. Shock heating caused by interactions between sub-relativistic jets and torus gas generates a compact SO evaporation region, smaller than 0.45 pc. Clumps containing evaporated SO are carried outward by the jet, forming outflows (blue arrows). Some of the clumps fall back onto the equatorial plane and eventually infall toward the SMBH (red arrows). The longer line-of-sight path through the SO-rich region toward the central receding component explains its high optical depth. Alt text: A cartoon showing the possible environment in the torus and jets in NGC 1052.